Might depend on the layout of the mowing. Pretty much open space and even terrain, go for the bigger deck. Otherwise I'd probably get the 48, still plenty of width and maybe a bit more user friendly. Mower should handle either with ease.
The difference is you need to do 9 runs on the 48" deck to cover the same ground as 8 runs on the 54" deck.
The time difference is marginal particularly if you end up having to mow slower with the bigger deck.
Add to that the bigger deck will scalp a lot worse then the smaller deck.
Now if you have a dead flat grassed area with no trees or other obstacles the bigger deck will do the same job saving you 1/9th of the time , ~10 minutes